Standing wave electron linear accelerators are widely used in radiation treatment. It has been a research direction over the past thirty years to extend the operating energy range, that is, to increase the output dosage over middle-energy and high-energy accelerators and to implement multiple purposes on one machine. The “Image Guided Radiation Treatment” (IGRT) is primary research direction in recent years. The related patents are as below:
1. U.S. Pat. No. 4,286,192 A, Tanabe et al., Varian, August 1981;
2. U.S. Pat. No. 4,382,208 A, Meddaugh et al., Varian, May 1983;
3. U.S. Pat. No. 4,629,938 A, Whitham, Varian, December 1986;
4. U.S. Pat. No. 4,746,839 A, Kazusa et al., NEC, May 1988;
5. U.S. Pat. No. 5,821,694 A, Young, LANL, October 1998;
6. U.S. Pat. No. 6,366,021 B1, Meddaugh et al., Varian, April 2002;
7. PCT/GB00/03004, Allen et al., Elekta, August 2000;
8. CN 1237079 A, TONG Dechun et al., TSINGHUA UNIVERSITY et al., December 1999.
When treading deceases using radiation treatment devices, the high-energy radiation beams radiated by electron linear accelerator are used to kill ill cells such as cancer cells. However, the energy of such radiation beams are much higher than that required by medical imaging. Therefore, what is needed is a device capable of switching between high energy and low energy such that the linear accelerator outputs low-energy electron beams when the radiation treatment device is used for examining, while outputs high-energy electron beams when the device is used for treating.
In the 20 cm beam focus segment in front of the electron linear accelerator, the electrons are accelerated to a velocity very close to the velocity of light (the energy is at about 1-1.5 MeV), in the following light segments the electrons are further accelerated over the wave to a high energy. Finally, the performance of the electron beams is determined by the relationship of field intensity and phase velocity to a great extent. The phase velocity, however, is a structural parameter, while the field intensity is changed over the power. The energy of electrons is decreased over the along with the decrease of power. When the power is decreased to a certain value, the relationship of field intensity and phase velocity in the beam focus segment goes far away from the design value, the performance of electron beam output is seriously deteriorated and trapping is greatly reduced so that the accelerator cannot function normally.
This problem can be avoided by using a phase switch to adjust energy. Assume that the electron beam energy finally output by the accelerator is 18 MeV, a phase switch is placed at a position when the electron energy reaches 12 MeV. When the phase switch is working, the accelerating segments after the switch are phase inversed, i.e., with a change of 180 degree in phase. Then the electrons are decelerated rather than being accelerated, with the energy decreased to 6 MeV from 12 MeV. Since the relationship of the field intensity and phase velocity in these two statuses is not changed, the 6 MeV electron beam has a performance as good as that of the 18 MeV electron beam.
Tanabe taught a design in U.S. Pat. No. 4,268,192 that, in a common side-coupling cavity, an end could be replaced by a movable piston. When the piston is extended into the coupling cavity, the frequency of TM011 or TEM modes is decreased to a value in wave band S and the structure is resonated again. The phases of the accelerating segments after the cavity change 180 degree and implement phase inversion because there's an additional phase movement of π in this coupling cavity. However, under this status, the field intensity in the coupling cavity is very high, and any moving components would cause high-frequency fire striking. During phase inversion, it is difficult to adjust field intensity separately. In addition, the structure is not operating in π/2 mode in this segment. A minor change of the position of the piston would not only affect the resonance capability of the whole structure, but also change the distribution of the field intensity.
In the above patent applications No. U.S. Pat. No. 4,286,192, U.S. Pat. No. 4,382,208, U.S. Pat. No. 4,629,938, and U.S. Pat. No. 6,366,021 obtained by Varian, the patent application No. U.S. Pat. No. 4,629,938 has always been used in the medical use accelerators produced by Varian. The Patent No. CN 1,237,079 A obtained by Tsinghua University is similar to the above patents. The technology of Tsinghua's is used in axis-coupling standing wave structure, while the technologies of Varian's are used in side-coupling standing wave structure. Patent No. U.S. Pat. No. 6,366,021 is the latest one. The above patents are all adjusting mechanisms used in a coupling cavity that adjust the relative field intensity in the previous and next accelerating structures by changing its coupling to the two adjacent accelerating cavities to improve the outputs at low-energy end. Therefore, they are often referred to as “energy switch”. The patent by NEC uses two predetermined coupling cavities that have different coupling to adjacent accelerating cavities, and achieves the same by deresonate one of the two cavities. However, all the technologies above improve the performance of the low-energy electron beam outputted by the accelerator by changing coupling coefficients to increase the field intensity of the beam focus segment, while do not incorporate phase inversion. Further discussions are omitted herewith.
The patent application No. PCT/GBOO/03004 by Elekta implements phase inversion by using a cylindrical coupling cavity having an axis perpendicular to the axis of the accelerator (conventionally, the axis of the coupling cavity is in parallel to the axis of the accelerator). The device operates in TE111 polarized mode, continuously adjusts its relative coupling to the adjacent accelerating cavities by rotating the polarization plane of the mode with mechanism, and achieves the purpose for phase inversion. However, according to the recitations in the description of this patent application, the frequency of the cylindrical coupling cavity would change when the polarization plane rotates so that the performance of the structure and the stability of operation are affected. Besides, since the device operates under a high order mode TE111, it may be easily affected by other adjacent high order modes during operation. Since there's still field intensity existed in the cylindrical coupling cavity, the device is not strictly operating in π/2 mode and also has the problem of fire striking. All these problems affect the operation stability of the device. In addition, the adjusting in the adjustment mechanism is not convenient, and has a low flexibility.